IgE antibodies are likely important in protective immunity to filaria and other helminths. Techniques have been developed to define these antibodies both quantitatively and qualitatively. Patients with the tropical eosophilia syndrome have enormous elevations of specific anti-filarial IgE antibody, while those with microfilaremia have the low levels, a finding apparently reflecting not qualitative differences in allergen recognition but differences in immuno-regulation of IgE antibody production. All patients have sufficient IgE to mediate allergic reactions to the parasite but paradoxically almost none have such reactions. IgG "blocking antibodies" appear to develop as a primary modulating mechanism of allergic responsiveness in these patients. The subclass restriction of these IgG antibodies is being defined. Because present clinical, parasitologic- or serologic methods are inadequate to diagnose many patients with filariasis, studies have focused on evaluating cross reactivity among helminth parasites and identifying with a variety of immunochemical techniques unique antigens that are parasite stage or species-specific. The findings have been utilized to develop other assays capable of detecting parasite antigen in the sera of infected individuals.